Photoamplifier arrangement



. 8, 1936. K. SCHLESINGER PHOTOAMPLIFIER ARRANGEMENT Filed Feb. 19, 1934 Jvren/or m WM Patented Dec. 8, 1936 2,083,195 PHOTOAMPLIFIER ARRANGEMENT Kurt Schlesinger, Berlin, Germany Application February 19, 1934, Serial No. 711,915 In Germany. February 20, 1933 8 Claims.

The invention relates to photo-electric arrangements for television purpose including a photo-electric cell and an amplifier connected thereto. The known arrangements oi this kind have a relatively high disturbing capacity between the cathode and the control grid of the amplifier tube in the order of more than 5.55 x 10 mi.

The effect of this disturbing capacity reducing the amplification of the high frequencies is known per se and therefore resulting a nonlinear amplification characteristic for the frequency.

It has already been proposed to connect the cathode of the photo-cell with the. grid of an after-connected amplifier tube. It has also been proposed to combine the photo-cell with the amplifier tube to form one structural unit. In these arrangements, however, the cathode of the photo-cell has always been connected with the grid of the amplifier. employed tubes with relatively high anode reactive efiect.

It is not possible in these previously known arrangements to reduce the capacity of the photo-cell arrangement to less than 5.55 x 10*" mf. In order to keep sufliciently small the varying influence of the disturbing capacity on the amplification gain for different frequencies, the ohmic resistance on which the photo-cell acts must be made very small (approximately 5000 ohms in the case of a -line image). When using such a small anode resistance, the potentials produced by the photo-cell are not great enough in comparison to the natural noise'of the amplifier, so that the production of television images with a large number of lines (for example 180 lines and. more) is practically impossible with arrangements of this nature.

The object of the invention is a photo-cell arrangement, the capacity of which is considerably less than 5.55 x 10- mf. (for example amounts to approximately 2.22 x 10- mf.) and which, therefore, permits of the production of television images of the kind in question without special, complicated means (carrier current method).

According to the invention 7 (1) there is employed as amplifier tube a tube which does not possess an internal capacity between anode and control grid (screening grid tube), having its grid cathode capacity reduced to the smallest possible extent, by renouncing a high amplification factor, so that the tube op- As amplifiers there have been crates with a little potential amplification degree (potential amplification 1:1),

(2) the cathode of the photo-cell is connected to earth by a condenser which enables free transmission of the generated photo-cell alternating 5 currents to earth, and the anode, whilst using the shortest possible lines and a working resistance as low in capacity as possible, is connected with the grid of the amplifier tube without the use of condensers.

' The disturbing total capacity of the arrangement according to the invention consists of the capacity of the photo-anode against earth, the inner photo-cell capacity (which is of extremely small amount if the anode is made sufiiciently 5 small), the assembly capacity of the leads and the working resistance, and the input capacity of the amplifier tube. Since the input capacity of the amplifier may also be very greatly reduced by the use of very short and very wide-mesh grids disposed as far away as possible from the cathode and the anode, it is possible with this arrangement according to the invention-in which photo-cell and amplifier system are arranged in different valves-to obtain a considerable reduction in the disturbing capacities hitherto capable of being accomplished. A still further reduction in the disturbing capacity may be obtained according to the invention by means of arranging photo-cell and amplifier in one single vacuum and constructing the anode of the photo-cell as part of the amplifier grid.

In this manner it is possible to make the natural capacity of the photo-anode and of the leads practically equal to 0. 35

' With the separate assembly of photo-cell, tube and resistance it is possible to reduce the total disturbing capacity to approximately 2.22 x 10 mf. It is possible, therefore, in the case of a 180-line image, with a maximum drop of 8% 40 at the highest frequencies, to operate with an anode resistance of 15,000 ohms. Since, however, the after-connected amplifier cascade lifts the high frequencies, there may be employed in practice an anode resistance of approximately 30,000 ohms. With an anode resistance of this value the sensitiveness of the photo-cell arrangement is already fully adequate for operations without interference.

When assembling the system in one evacuated 50 envelope the anode resistance may, in accordance with the invention, also be provided in the tube itself, and owing to the reduction of the disturbing capacities may be selected to be still larger. The same, for example, with a drop of 5 merely 8% (in a 180-line image) may amount to 20,000-25,000' ohms. Practically, therefore, as set forth above, a resistance of approximately twice the size, i. e., up to approximately 50,000 ohms, may be employed, so that the eflectiveness of the connection system is nearly ten times greater than that of the previously known arrangements.

By the circuit arrangement according to the invention it is effected that both the capacity of the photo-cathode (which is practically equal to the radius of the sphere of the cell and, therefore, cannot be made smaller than approximately 2.22 x 10- ml.) as well as the internal capacity of the amplifier tube do not increase considerably the disturbing capacity, and that therefore the remaining grid-cathode capacity is substantially smaller than in the case of the known arrangements.

Fig. 1 shows a possible form of embodiment. Fig. 2 shows an arrangement in which the photocell and the first amplifier stage and the working resistance are provided in the same evacuated envelope.

Fig. 1 shows the photo-cell I with the-anode and the cathode 3, 4 the grid, the anode l0 and the cathode 20 of the first amplifier tube and the anode resistance 3 of the photo-cell circuit. The photo-cell circuit is accordingly completed over the photo-cell, resistance 5 and return condenser 6 of approximately 1 mf., while the photo-cell receives itsbias from a battery 8 over a resistance 'Lof approximately 1 megohm. The line from the cell to the tube is made as short as possible. The capacity 9 of the grid of the tube is reduced by the fact that the grid is made very short and with wide mesh, and is mounted as far as possible away from the cathode and from the anode [0. The capacity of the reaction may be overcome entirely by employing a screening grid tube. In this connection the amplification degree may be reduced to such extent until, with a resistance R8 (II), a potential amplification gain=1 is obtained. The anode current source I5 is connected over the resistance II with the anode l0 and over a resistance IS with the screen grid i6. By-pass condensers l2 and H are connected parallel with the source l5 and the resistance I6.

Fig. 2 shows an embodiment of the invention, in which the. photo-cell and the first amplifier system are provided in the same vacuum.

In the drawings I1 is the common vessel, 3 the photo-electric layer provided on the upper part of the wall of the vessel with the lead l8, and I9 a window aperture, through which the light falls on to the photo-electric layer 3. In the lower part of the bulb there is provided the amplifier system, which comprises the grid 4, the indirectly heated cathode 20, the screening grid I6 and the anode ID. All electrodes may conveniently be secured to a support 2! consisting of insulating material. The end of the grid 4 located in the upper space of the vessel acts as the anode of the photo-cell. At this end there may conveniently be provided a cap 22, which contains the metal for the production of the photo-layer 3, or if desired a suitable reaction mixture for developing'this metal. The lower end of the grid is connected over the resistance 5 with the cathode 20. In accordance with the invention the diameter of this cap may be made very small. Naturally it is also possible to mount the cap completely separate from the grid, viz., in such fashion, that not form a part of the electrical system.

The cap may be filled, for example, with a mixture of bichromate of caesium and silicon. If the cap is heated to the reaction temperature of the mixture in the manner known per se, for example by means of eddy currents, pure metallic caesium is developed, which is deposited in the upper part of the bulb. The part of the bulb intended for the formation of the photolayer may preferably be furnished with a suitable preliminary coating, for example of metallic silver or silver oxide.

In accordance with the invention, there may be provided above the glass support 2| a reflector 23, which is composed for example of metallized mica and does not require to be earthed. This reflector causes in itself reflection of the light and, particularly if the window i0 is provided centrally over the axis of the tube, results in a considerably better utilization of the entering light.

It is desirable in accordance with the invention at first to place the amplifier system in an entirely operable condition (outgassing of the electrodes, atomization of highly emissive metal on to the cathode or forming of the oxide cathode), and not until then-in the well evacuated bulbto atomize the metal (for example caesium) serving for the production of the photoelectric layer.

For many purposes of use it is desirable to provide several or all of the after-connected amplifier systems of the cascade in the same bulb, as in this manner the total capacity of the cascade can be considerably reduced.

'I claim:

1. In a photo-amplifier arrangement for television purposes a. photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the cathode of said photo-cell connected with the cathode of said amplifier tube over a capacity in the order of 1 microfarad and with the negative pole of a potential source, the positive pole of said potential source connected with the cathode of said amplifier tube, the anode of said amplifier tube connected with an output circuit and with an anode current source, the screen grid of said amplifier tube connected with the positive pole of a potential source over a resistance.

2. In a photo-amplifier arrangement for television purposes a photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the,

it does tive pole of a potential source over a resistance,

the control grid of said amplifier tube being very short, having wide mesh and having relatively Great distance from the anode and the cathode.

3. In a photo-amplifier arrangement for television purposes a photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the cathode of said photo-cell connected with the cathode oi! said amplifier tube over a capacity in the order of one microiarad and with the negative pole of a potential source, the positive pole of said potential source connected with the cathode of said amplifier tube, the anode of said amplifier tube connected with an output circuit and with an anode current source, the screen grid of said amplifier tube connected with the positive pole of a potential source over a resistance, the control grid of said amplifier tube being very short, having wide mesh and having relatively great distance from the anode and the cathode, said resistance having small natural capacity.

4. In a photo-amplifier arrangement for television purposes a photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid 01 said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the cathode of said photo-cell connected with the cathode of said amplifier tube over a capacity in the order of l microfarad and with the negative pole of a potential source, the positive pole of said potential source connected with the cathode of said amplifier tube, the anode of said amplifier tube connected with an output circuit and with an anode current source, the screen grid of said amplifier tube connected with the positive pole of a potential source over a resistance, the control grid of said amplifier tube being very short, having wide mesh and having relatively great distance from the anode and the cathode, said resistance having small natural capacity, and a resistance in the order of 30,000 ohms.

5. In a photo-amplifier arrangement for television purposes a photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the oathode oi said photo-cell connected with the cathode of said amplifier tube over a capacity in the order of l mlcrofarad and with the negative pole of a potential source, the positive pole of said potential source connected with the cathode of said amplifier tube, the anode of said amplifier tube connected with an output circuit and with an anode current source, the screen grid of said amplifier tube connected with the positive pole of a potential source over a resistance, the control grid of said amplifier tube being very short, having wide mesh and having relatively great distance from the anode and the cathode, said photo-cell and said amplifier stage assembled in a common vacuum tube.

6. In a photo-amplifier arrangement for television purposes a photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode 01' said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the cathode of said photo-cell connected with the cathode of said amplifier tube over a capacity in the order of 1 microi'arad and with the negative pole of a potential source, the positive pole of said potential source connected with the cathode of said amplifier tube, the anode of said amplifier tube connected with an output circuit and with an anode current source, the screen grid of said amplifier tube connected with the positive pole of a potential source over a resistance, the control grid of said amplifier tube being very short, having wide mesh and having relatively great distance from the anode and the cathode, said photo-cell and said amplifier stage assembled in a common vacuum tube, said working resistance being in the order of 50,000 ohms.

7. In a photo-amplifier arrangement for television purposes a photo-electric cell including a cathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the cathode of said photo-cell connected with the cathode of said amplifier tube over a capacity in the order of 1 microfarad and with the negative pole of a potential source, the positive pole of said potential source connected with the cathode 01' said amplifier tube, the anode of said amplifier tube connected with an output circuit and with an anode current source, the screen grid of said amplifier tube connected with the positive pole of a potentiometer source over a resistance, the control grid of said amplifier tube being very short, having wide mesh and having relatively great distance from the anode and the cathode, said photo-cell and said amplifier stages assembled in a common vacuum tube, said working resistance being in the order or 50,000 ohms, and included in said common vacuum tube.

8. In a photo-amplifier arrangement for television purposes a photo-electric cell including acathode and an anode and an amplifier valve including a cathode, a control grid electrode, a screening grid and an anode, the anode of said photo-cell having a small surface connected directly with the control grid of said amplifier tube by a short lead and with the cathode of said amplifier tube by a working resistance, the cathode of said photo-cell connected with the cathode of said amplifier tube over a capacity, a potential source connected with said photo-cell, the anode of said amplifier tube connected with an output circuit, an anode current source, a potential source for efiecting a screen grid bias.

KURT SCHLESINGER. 

